David Hill wrote:
Nick says it's got something, but they're not greenfly. Apparently it's a
pear disease. Don't ask me how that works. He says the leaves go all red
and blistered, and there's some kind of insect causing it.

Could it be pear leaf blister mite?
http://apps.rhs.org.uk/advicesearch/...e.aspx?PID=661

That may be what Nick thinks it is, but it doesn't look like it is
something carried on plum trees. *shrug*